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  • de Jonge, Lis  (55)
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  • 1
    Language: English
    In: Soil Science Society of America Journal, Nov-Dec, 2013, Vol.77(6), p.1965(12)
    Description: Soil structure maintains prime importance in determining the ability of soils to carry out essential ecosystem functions and services. This study quantified the newly formed structure of 22-mo field-incubated physically disturbed (2-mm sieved) samples of varying clay mineralogy (illite, kaolinite, and smectite) amended with organic material (7.5 Mg ha-1). The newly formed structure was compared with that of sieved, repacked (SR) and natural intact samples described in previous studies. Assessment and comparison of structural complexity and organization was done using water retention (pore size distribution), soil gas diffusivity, air permeability, and derived pore network complexity parameters. Significant decreases in bulk density and increases in pores 〉100 ?m were observed for incubated samples compared with SR samples. For the soils studied, the proportion of pores 〉100 ?m increased in the order: smectite 〈 illite 〈 kaolinite, with no effect of organic amendment. Soil structural complexity, quantified by soil gas diffusivity, air permeability, and derived pore network indices, was greater for incubated than SR samples. For illitic soils, incubated samples had lower water content and higher air-filled porosity and air permeability than natural intact samples at a matric potential of -10 kPa. Despite this, soil pore organization was similar for both natural and incubated soils, but pore network complexity increased in the order: SR 〈 incubated 〈 natural soils. Finally, the air permeability percolation threshold corresponding to the physically based diffusion threshold increased with structural complexity (SR = 0.02 ?m2; incubated = 0.20 ?m2; natural = 0.70 ?m2). Thus, critical reexamination is needed of the often-used 1.0-?m2 percolation threshold for convective air transport when analyzing pore network complexity. Lack of a clear effect of organic amendment for incubated samples suggests using higher application rates in future studies.
    Keywords: Porosity -- Analysis ; Soil Permeability -- Analysis ; Soil Research
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 2
    Language: English
    In: Journal of Hydrology, February 2015, Vol.521, pp.498-507
    Description: The present study proposes a new two-step approach to prediction of the continuous soil water characteristic (SWC) from saturation to oven-dryness from a limited number of measured textural data, organic matter content and dry bulk density. The approach combines dry- and wet-region functions to obtain the entire SWC by means of parameterizing a previously developed continuous equation. The dry region function relates gravimetric soil fractions to adsorptive forces and the corresponding water adsorbed to soil particles. The wet region function converts the volumetric particle size fractions to pore size fractions and utilizes the capillary rise equation to predict water content and matric potential pairs. Twenty-one Arizona source soils with clay and organic carbon contents ranging from 0.01 to 0.52 kg kg and 0 to 0.07 kg kg , respectively, were used for the model development. The SWCs were measured with Tempe cells, a WP4-T Dewpoint Potentiameter, and a water vapor sorption analyzer (VSA). The model was subsequently tested for eight soils from various agricultural fields in Denmark with clay contents ranging from 0.05 to 0.41 kg kg . Test results clearly revealed that the proposed model can adequately predict the SWC based on limited soil data. The advantage of the new model is that it considers both capillary and adsorptive contributions to obtain the SWC from saturation to oven-dryness.
    Keywords: Capillarity ; Adsorption ; Unsaturated Soil ; Water Retention ; Soil Moisture ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 3
    Language: English
    In: Soil Science Society of America Journal, March-April, 2014, Vol.78(2), p.377(10)
    Description: Quantitative characterization of aggregate pore structure can reveal the evolution of aggregates under different land use and management practices and their effects on soil processes and functions. Advances in X-ray computed tomography (CT) provide powerful means to conduct such characterization. This study examined aggregate pore structure of three differently managed same textured Danish soils (mixed forage cropping, MFC; mixed cash cropping, MCC; cereal cash cropping, CCC) for (i) natural aggregates, and (ii) aggregates regenerated after 20 mo of incubation. In total, 27 aggregates (8-16 mm) were sampled from nine different treatments; three natural soils and three repacked lysimeters without and three with organic matter (OM; ground rape) amendment. Three dimensional X-ray CT images, tensile strength, and organic carbon (OC) were obtained for each aggregate. Aggregate-associated OC differed significantly between the three soils as 2.1, 1.4, and 1.0% for MFC, MCC, and CCC, respectively. Aggregate porosity and pore connectivity were significantly higher for CCC aggregates than for MFC and MCC aggregates. The CCC aggregates had an average pore diameter of 300 ?m, whereas MFC and MCC had an average pore diameter of 200 and 170 ?m, respectively. Pore shape analysis indicated that CCC and MFC aggregates had an abundance of rounded and elongated pores, respectively, and those of MCC were in-between CCC and MFC. Aggregate pore structure development in the lysimeters was nearly similar irrespective of the soil type and organic matter amendment, and was vastly different from the state of natural aggregates. Aggregate porosity (〉30 ?m) was observed to be a good predictor for the mechanical properties of aggregates. In general natural aggregates were stronger than lysimeter aggregates.
    Keywords: Cat Scans -- Usage ; Porosity -- Research ; Soil Research
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 4
    Language: English
    In: Soil Science Society of America Journal, March-April, 2014, Vol.78(2), p.422(12)
    Description: Changes in soil water content are known to affect soil reflectance. Even though it was suggested some time ago that the phenomenon of increased forward scattering due to the presence of water in the soil is related to water film thickness and matric potential, there has been no detailed investigation of this in any studies. The effects of moisture conditions on visible near-infrared (vis-NIR) spectra of four representative soils in Denmark have been assessed as a function of both water film thickness (expressed as the number of molecular layers) and matric potential. Complete water retention curves, from wet (pF 0.3, pF = log(|j|), where ? is the matric potential in cm) to hyper dry end (oven-dried and freeze-dried soil), were obtained by initial wetting followed by successive draining and drying of soil samples, performing NIR measurements at each step. Soil reflectance was found to decrease systematically, yet not proportionally, with decreasing matric potential and increasing molecular layers. The changes in molecular layers were best captured by the soil reflectance of clay-rich soils. Here the largest increase in reflectance occurred between pF 3 and 4, caused by the shift from capillary to adsorptive surface forces. In support of this, the smallest changes in reflectance were seen in the sandiest soil. Freeze drying the soil highest in organic C increased reflectance, possibly due to an alteration in organic matter during freezing. The different reflectance behavior of soil with a higher organic C content may be linked to differences in the amount, but also the quality (higher hydrophobicity) of the organic matter. However, this needs to be confirmed in further studies.
    Keywords: Reflectance -- Research ; Molecules -- Research ; Soil Research
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 5
    Language: English
    In: Soil Science Society of America Journal, Sept-Oct, 2013, Vol.77(5), p.1517(12)
    Description: Time domain reflectometry (TDR) is used widely for measuring soil-water content. New TDR coil probe technology facilitates the development of small, nondestructive probes for simultaneous measurement of soil-water content (?) and soil-water potential (?). In this study we developed mini tensiometer-time domain reflectometry (T-TDR) coil probes, 6-mm wide and 32-mm long. The coil probes were calibrated against a conventional three-rod probe and were used for measuring ? for a aggregated volcanic ash soil (VAS) and a uniform sand. A commonly-used dielectric mixing model did not accurately describe the measured relation between the dielectric constant of the T-TDR coil probe (?coil) and ?, and a new calibration model for ?coil (?) was proposed instead. The new model assumes single-region behavior for sand and two-region behavior for aggregated VAS, when plotting the normalized dielectric constant of the coil probe (?coil-? dry; where ?dry is the dielectric constant of the T-TDR coil probe for air-dried material) as a function of ?. The new calibration model accurately described the (?coil-? dry)-? relations measured by 7 T-TDR coil probes on both sand and VAS. Additionally, there was a good agreement between measured soil-water retention curves (? 〉 -100 cm H2O) by the new T-TDR coil probes and independent measurements by the hanging water column method.
    Keywords: Reflectometers -- Usage ; Soil Research
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 6
    Language: English
    In: Soil Science Society of America Journal, Nov-Dec, 2013, Vol.77(6), p.1944(11)
    Description: The time dependency of water repellency (WR) in hydrophobic porous media plays a crucial role for water infiltration processes after rainfall and for the long-term performance of capillary barrier systems. The contact angle (CA) of hydrophobic media normally decreases with continuous contact with water, eventually allowing water imbibition. However, the effect of the reduction in CA with soil-water contact time on the water retention function of hydrophobic media is not yet fully understood. In this study, water retention characteristics were measured using a hanging water column apparatus equipped with a mini-time domain reflectometry (TDR) coil probe under controlled wetting and drying in a water-repellent volcanic ash soil (VAS) and in sands coated with different hydrophobic agents. The contact angle (CA-SWRC) under imbibition was evaluated based on the inflection points on the water retention curves. For both water-repellent VAS and hydrophobized sand samples, the calculated CA-SWRC increased with increasing WR. This was determined from both the water drop penetration time and the initial contact angle (CAi) by the sessile drop method. Calculated CAuSWRC values ranged from 20[degrees] to 48[degrees] for the water-repellent VAS and from 40[degrees] to 63[degrees] for the hydrophobized sand. The CA reduction with contact time was quantified by relating CA-SWRC and CAi. This gave a significant linear relationship for the hydrophobized sand [CA-SWRC = 0.40CAi +11.3 (30[degrees] 〈 CAi 〈 120[degrees]), R2 = 0.66]. A large difference in water-filled pore distributions under controlled wetting and drying cycles was found on c
    Keywords: Water Repellents -- Usage ; Hydrophobic Effect -- Analysis ; Soil Research
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 7
    Language: English
    In: Soil Science Society of America Journal, May-June, 2014, Vol.78(3), p.754(7)
    Description: The characterization and description of important soil processes such as water vapor transport, volatilization of pesticides, and hysteresis require accurate means for measuring the soil water characteristic (SWC) at low water potentials. Until recently, measurement of the SWC at low water potentials was constrained by hydraulic decoupling and long equilibration times when pressure plates or single-point, chilled-mirror instruments were used. A new, fully automated vapor sorption analyzer (VSA) helps to overcome these challenges and allows faster measurement of highly detailed water vapor sorption isotherms. In this technical note we present a comprehensive evaluation of the VSA instrument for a wide range of differently textured soils and discuss optimal measurement settings. The effects of operation mode, air-flow rate, sample pretreatment, test temperature, sample mass, and mass trigger point on resultant sorption isotherms were evaluated for a relative humidity (RH) range from 0.10 to 0.90. Both adsorption and desorption branches were measured for all soils within a reasonable time period (10-50 h). Sample masses larger than 3.5 g resulted in incomplete adsorption and desorption, while oven-dry (105[degrees]C) samples of coarse-textured soils exhibited water repellency characteristics. The required measurement times were strongly correlated with clay content and influenced by high organic carbon content.
    Keywords: Water Vapor -- Measurement ; Volatilization (Physics) -- Research ; Soil Research
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 8
    Language: English
    In: Soil Science Society of America Journal, March, 2013, Vol.77(2), p.362(10)
    Description: Understanding soil-gas phase properties and processes is important for finding solutions to critical environmental problems such as greenhouse gas emissions and transport of gaseous-phase contaminants in soils. Soil-air permeability, ka (?m2), is the key parameter governing advective gas movement in soil and is controlled by soil physical characteristics representing soil texture and structure. Models predicting ka as a function of air-filled porosity (?) often use a reference-point measurement, for example, ka,1000 at ?1000 (where the measurement is done at a suction of -1000 cm H2O). Using ka measurements from two Danish arable fields, each located on natural clay gradients, this study presents a pore tortuosity-disconnectivity analysis to characterize the soil-gas phase. The main objective of this study is to investigate the effect of soil-moisture condition, clay content, and other potential drivers of soil texture and structure on soil-gas phase characteristics based on a ka-based pore tortuosity parameter, Xa [= log(ka/ka,1000)/log(?/?,1000)]. Results showed that Xa did not vary significantly with soil matric potential (in the range of -10 to -1000 cm H2O), but the average Xa across moisture conditions showed a strong linear relation (R2 = 0.74) to clay content. The Xa, further showed promising relations to specific surface area, Rosin-Rammler particle size distribution indices, ? and ? (representing characteristic particle size and degree of sorting, respectively), and the Campbell water retention parameter, b. Considering clay as a main driver of soil-gas phase characteristics, we developed expressions linking clay content and ka,1000 at ?1000 and discussed the effect of clay content on general ka-? behavior.
    Keywords: Grading (Building materials) -- Usage ; Soil Permeability -- Analysis ; Soil Research
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 9
    Language: English
    In: Soil Science Society of America Journal, 2013, Vol.77(2), pp.403-411
    Description: The influence of clay content in soil-pore structure development and the relative importance of macroporosity in governing convective fluid flow are two key challenges toward better understanding and quantifying soil ecosystem functions....
    Keywords: Life Sciences ; Agricultural Sciences ; GAS ; Models ; Consequences ; Porosity ; Air Permeability ; Parameters ; Transport ; Microtomography ; Quantification ; Agriculture
    ISSN: 0361-5995
    E-ISSN: 1435-0661
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  • 10
    Language: English
    In: Soil Science Society of America journal, 2012, Vol.76(6), pp.1946-1956
    Description: Modeling water distribution and flow in partially saturated soils requires knowledge of the soil water characteristic (SWC). However, measurement of the SWC is challenging and time-consuming and, in some cases, not feasible. This study introduces two predictive models (F(W)–model and A(W)–model) for the SWC, derived from readily available soil properties such as texture and bulk density. A total of 46 undisturbed soils from different horizons at 15 locations across Denmark were used for model evaluation. The F(W)–model predicts the volumetric water content as a function of volumetric fines content (organic matter and clay). It performed reasonably well for the dry-end of SWC (above a pF value of 2.0; pF = log(|ψ|), where ψ is the matric potential in cm), but did not do as well closer to saturated conditions. The A(W)–model predicts the volumetric water content as a function of volumetric content of different particle size fractions (organic matter, clay, silt, and fine and coarse sands). The volumetric content of a particular soil particle size fraction was considered if it contributed to the pore size fraction still occupied with water at the given pF value. Hereby, the A(W)–model implicitly assumes that a given particle size fraction creates an analogue pore size fraction and further this pore size fraction filled with water is corresponding to a certain pF value according to the well-known capillary rise equation. The A(W)–model was found to be quite robust, and it performed exceptionally well for pF values ranging from 0.4 to 4.2 for different soil types. For prediction of the continuous SWC, it is recommended to parameterize the van Genuchten model based on the SWC data points predicted by the A(W)–model. ; p. 1946-1956.
    Keywords: Clay ; Particle Size ; Soil Types ; Bulk Density ; Texture ; Prediction ; Capillarity ; Silt ; Equations ; Organic Matter ; Models ; Water Distribution ; Soil Water Characteristic ; Water Content ; Saturated Conditions
    ISSN: 0361-5995
    E-ISSN: 14350661
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